Ammonium nitrate conventionally is manufactured by reacting nitric acid and ammonia feeds that are independently supplied from separate sources. The manufacturing process typically involves reacting the nitric acid and ammonia in a neutralisation vessel or, more usually, in a pipe reactor and, in the interest of producing concentrated ammonium nitrate solution (for example of the order of 80% concentration), nitric acid having a concentration of 50% to 68% or greater typically is employed in the process. However, as is well understood in the industry, this level of acid concentration leads to a violent exothermic reaction, this in turn imposing rigorous demands on processing apparatus.
The nitric acid component conventionally is manufactured by the Ostwald Process which, in basic reaction terms, involves a three-stage process comprising, firstly, oxidising ammonia in the presence of air by effecting rapid high temperature catalytic conversion of the ammonia-air mixture to produce nitrogen monoxide. The resultant stream is cooled (usually under pressure) and some of the nitrogen monoxide reacts non-catalytically with oxygen to form such higher oxides of nitrogen as nitrogen dioxide and its dimer; the mixture of which is referred to below as nitrogen dioxide and the resultant stream as a whole being referred to as nitrous gas. Following further cooling, the nitrous gas is admitted to a counter-flow absorption process to produce the nitric acid.
The absorption process is performed within an absorption tower, with the product acid concentration typically being between 50% and 68%, depending upon the operating pressure of and the number of absorption stages in the absorption tower, and the concentration of nitrogen oxides entering the absorption tower. The nitrous gases normally are relatively dilute because of the presence of nitrogen introduced in the air and, hence, a large absorption tower typically is required to facilitate reaction and absorption.
It has now been determined that, with substantial modification of the above described conventional nitric acid manufacturing process, including oxidation of the ammonia in the presence of oxygen, admission of water ballast prior to the ammonia oxidation stage, retention of the water ballast throughout the process and with acceptance of end product in the form of dilute nitric acid (for example having a concentration of the order of 20% to 40%), an absorption stage may be adopted that utilises heat exchange technology and which obviates the conventional requirement for an absorption tower and its attendant disadvantages. This then facilitates integration of the two (hitherto separate) processes of nitric acid production and subsequent ammonium nitrate production.